The present system relates generally to cardiac rhythm management systems and particularly, but not by way of limitation, to such a system using time-domain heart rate variability indicia.
When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body""s circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems are often implanted in the patient and deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via an intravascular leadwire or catheter (referred to as a xe2x80x9cleadxe2x80x9d) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as xe2x80x9ccapturingxe2x80x9d the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly. Such pacers coordinate atrial and ventricular contractions to improve pumping efficiency. Cardiac rhythm management systems also include coordination devices for coordinating the contractions of both the right and left sides of the heart for improved pumping efficiency.
Cardiac rhythm management systems also include defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Such defibrillators also include cardioverters, which synchronize the delivery of such stimuli to portions of sensed intrinsic heart activity signals. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn""t allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a defibrillation countershock, also referred to simply as a xe2x80x9cshock.xe2x80x9d The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other implantable or external systems or devices for diagnosing or treating cardiac arrhythmias.
One problem faced by physicians treating cardiovascular patients is assessing patient well-being for providing a prognosis or for adjusting therapy to improve the patient""s prognosis. Heart rate variability (xe2x80x9cHRVxe2x80x9d) is thought to provide one such assessment of cardiovascular health. The time interval between intrinsic ventricular heart contractions changes in response to the body""s metabolic need for a change in heart rate and the amount of blood pumped through the circulatory system. For example, during a period of exercise or other activity, a person""s intrinsic heart rate will generally increase over a time period of several or many heartbeats. However, even on a beat-to-beat basis, that is, from one heart beat to the next, and without exercise, the time interval between intrinsic heart contractions varies in a normal person. These beat-to-beat variations in intrinsic heart rate are the result of proper regulation by the autonomic nervous system of blood pressure and cardiac output; the absence of such variations indicates a possible deficiency in the regulation being provided by the autonomic nervous system.
The autonomic nervous system itself has two components: sympathetic and parasympathetic (or vagal). The sympathetic component of the autonomic nervous system is relatively slow acting, and is associated with a tendency to raise heart rate, blood pressure, and/or cardiac output. The parasympathetic/vagal component of the autonomic nervous system, which provides a relatively faster response than the sympathetic component, is associated with a tendency to reduce heart rate, blood pressure, and/or cardiac output. A proper balance between the sympathetic and parasympathetic components of the autonomic nervous system is important. Therefore, an indication of this balance of the components of the autonomic nervous system, which is sometimes referred to as xe2x80x9cautonomic balance,xe2x80x9d xe2x80x9csympathetic tone,xe2x80x9d or xe2x80x9csympathovagal balance,xe2x80x9d provides a useful indication of the patient""s well-being.
One technique for providing an indication of the balance of the components of the autonomic nervous system is provided by the beat-to-beat heart rate variability, as discussed above. More particularly, intrinsic ventricular contractions are detected. The time intervals between these contractions, referred to as the R-R intervals, are recorded after filtering out any ectopic contractions, that is, ventricular contractions that are not the result of a normal sinus rhythm. This signal of R-R intervals is typically transformed into the frequency-domain, such as by using fast Fourier transform (xe2x80x9cFFTxe2x80x9d) techniques, so that its spectral frequency components can be analyzed. Two frequency bands are of particular interest: a low frequency (LF) band in the frequency (xe2x80x9cfxe2x80x9d) range 0.04 Hzxe2x89xa6f less than 0.15 Hz, and a high frequency (HF) band in the frequency range 0.15 Hzxe2x89xa6fxe2x89xa60.40 Hz. The HF band of the R-R interval signal is influenced only by the parasympathetic/vagal component of the autonomic nervous system. The LF band of the R-R interval signal is influenced by both the sympathetic and parasympathetic components of the autonomic nervous system. Consequently, the ratio LF/HF is regarded as a good indication of the autonomic balance between sympathetic and parasympathetic/vagal components of the autonomic nervous system. An increase in the LF/HF ratio indicates an increased predominance of the sympathetic component, and a decrease in the LF/HF ratio indicates an increased predominance of the parasympathetic component. For a particular heart rate, the LF/HF ratio is regarded as an indication of patient wellness, with a lower LF/HF ratio indicating a more positive state of cardiovascular health.
Such spectral analysis of the frequency components of the R-R interval signal has required an FFT (or other parametric transformation, such as autoregression) transformation from the time domain into the frequency domain. Implantable cardiac rhythm management devices, however, typically do not presently have the dedicated hardware to perform such FFT transformations. Even if an implantable cardiac rhythm management device did have such dedicated FFT hardware, performing the transformation would be computationally expensive, requiring increased power consumption, and shortening time during which the implanted battery-powered device can be used before its replacement is required. Therefore, there is a need to provide such an indication of patient well-being without requiring a computationally expensive transformation of the R-R interval signal into the frequency domain.
This document describes a cardiac rhythm management system that provides an indication of patient well-being based on the autonomic balance between the sympathetic and vagal components of the autonomic nervous system, using time-domain processing of frequency components of a heart rate variability signal.
In one embodiment, the cardiac rhythm management system provides a method that detects heart contractions over a time period. A time-domain first signal represents time intervals between the detected heart contractions. The first signal is filtered to obtain a time-domain second signal including frequency components substantially in a first frequency band. The first signal is also filtered to obtain a time-domain third signal including frequency components substantially in a second frequency band that is different from the first frequency band. Based on the second and third signals, the system provides an indication associated with an autonomic nervous system.
In another embodiment, the cardiac rhythm management system provides a method that detects contractions of a heart over a time period. A time-domain first signal represents time intervals between the detected heart contractions. The first signal is filtered to obtain a time-domain second signal having frequency components substantially in a first frequency band. The system provides a substantially real-time time-domain indication, based on the second signal, of a balance between a sympathetic and a parasympathetic/vagal components of an autonomic nervous system. In a further embodiment, the system delivers therapy to a heart based on this indication of autonomic balance.
In another embodiment, the cardiac rhythm management system includes a heart contraction detection module, providing a heart rate interval signal carrying information regarding intervals between heart contractions. A bandpass filter is coupled to the detection module for receiving the heart rate interval signal. The bandpass filter provides a time-domain bandpass filtered signal output. A variance module is coupled to the bandpass filter for receiving the bandpass filtered signal. The variance modules provides a resulting variance signal. An autonomic balance indicator module is coupled to the variance module, and provides an indication of a balance between sympathetic and parasympathetic components of an autonomic nervous system, based on the variance signal. In a further embodiment, a therapy module, which is adapted to be coupled to a heart, provides therapy to the heart based at least in part on the autonomic balance indication. Other aspects of the invention will be apparent on reading the following detailed description of the invention and viewing the drawings that form a part thereof.